Major parts of San Francisco Bay’s shoreline are slowly sinking, a new scientific study has found, dramatically increasing the risk of billions of dollars of flooding in the coming decades as sea level rise continues due to climate change.

Much of the bay’s shoreline, because it is built on mud that compacts over time, is sinking at about 2 millimeters a year, roughly the thickness of a nickel, the study by researchers at UC Berkeley and Arizona State University found. But prominent areas that were built on fill that was not densely compacted, including sand, gravel, garbage and other debris — such as San Francisco International Airport, Treasure Island and Foster City — are sinking at a much faster rate, about 10 millimeters, or nearly half an inch a year. They face a far more serious risk of being underwater not generations from now but much sooner, according to the study. And the Bay Area isn’t ready.

“This is a big problem in the Bay Area,” said Roland Burgmann, a UC Berkeley professor of earth and planetary science and co-author of the study, published Wednesday in the journal Science Advances. “Quite a few of these areas are developed. In terms of long term planning, it’s a huge problem.”

Flood risk maps produced by FEMA, the Federal Emergency Management Agency, were computed using precise radar measurements from satellites. But the maps don’t take ground sinking, known as subsidence, into account. As a result, cities, counties, developers, insurance companies and home buyers are underestimating flood risks, not only decades from now when the bay’s level will be higher but also much sooner, when large storms with big tidal surges could devastate the low-lying properties in the coming years, as Hurricane Sandy did in 2012 in New York City.

San Francisco Bay’s waters already have risen 8 inches since the Gold Rush. A tide gauge at Fort Point, next to the Golden Gate Bridge, has recorded measurements since 1850.

And the pace is accelerating. The 10 hottest years on Earth since 1880 all have occurred since 1998, according to NASA, the National Oceanic and Atmospheric Administration and other federal agencies. As the temperature continues to rise, polar ice is melting, raising sea levels around the world. Warmer ocean temperatures also cause water volume to expand, generating more sea level rise.

The most recent studies by the National Academy of Sciences and United Nations estimate that depending on the amount of greenhouse gases put into the atmosphere in the coming years, the Pacific Ocean on the West Coast — and in turn, San Francisco Bay — is projected to rise up to a foot in 12 years, 2 feet in 32 years and 5 feet in 82 years.

Until now, planners and scientists only have been modeling how much flooding to expect from the sea level rise. They haven’t considered that as the ocean rises, some of the land around the bay’s shoreline is dropping at a similar, or faster, rate.

The former studies estimated that between 12,800 and 102,400 acres of bay shoreline are at serious risk of flooding by the year 2100. But when the impacts of the ground sinking are added in, the area threatened increases to between 30,720 and 106,240 acres, the new study found.

“There are many estimates and models for sea level rise,” said Manoochehr Shirzaei, assistant professor in Arizona State University’s School of Earth and Space Exploration and a member of NASA’s Sea Level Change planning team. “But they all fall short because they don’t take into account land elevation changes.”

Bay Area cities and counties have three choices, experts say. First, they can build wetlands in some areas, like the former Cargill salt evaporation ponds in the South Bay, and hay fields in the North Bay. Wetlands buffer waves and storms, reducing flood impacts on the shorelines.

Voters in 2016 approved $500 million in new funding over the next 20 years for bay wetlands restoration and flood control when they passed Measure AA, a $12-per-year parcel tax in all nine Bay Area counties. The first grants are scheduled to go out next month.

Second, cities can build concrete seawalls and levees. That will be the option for important features that cannot be moved, such as airports or the Embarcadero along the San Francisco waterfront. But it’s expensive.

Finally, some areas will be allowed to flood if the costs are too high to preserve them. “How much are we going to spend to defend the shoreline?” said Patrick Barnard, a research geologist with the U.S. Geological Survey in Menlo Park. “In all likelihood, parts of the Bay Area are going to look like New Orleans one day. It’s not in our nature to get out of the way and retreat. And that’s going to be very expensive. We’re talking about hundreds of billions of dollars.” Other vulnerable areas described in the study include Alviso in North San Jose and portions of the Hayward and Union City shoreline.

In Foster City, which was built on fill in the 1960s by developer Jack Foster, voters in June will be asked to approve a $90 million bond to raise the levees to reduce flood risk.

David Lewis, executive director of Save the Bay, an Oakland environmental group, said more must be done.“FEMA needs to update the Bay Area flood maps faster so we can actually prepare so that taxpayers don’t end up subsidizing risky development in flood prone areas unknowingly via federal flood insurance,” Lewis said.

Already, American cities have seen huge costs in recent years from coastal flooding. Hurricane Sandy caused $69 billion in damage on the East Coast as record- high storm surges destroyed seaside communities and sent floodwaters pouring down the stairs of the New York City subway system, causing blackouts when the water hit electrical equipment. Afterward, then-Mayor Michael Bloomberg outlined a $20 billion plan to protect low-lying Manhattan and other parts of the city with a network of levees and flood walls.

Around San Francisco Bay, the state’s Bay Conservation and Development Commission is working on studies with local communities to address sea level rise issues. The studies, which are scheduled for release in July 2019, will help guide planning in the years ahead. But the commission hasn’t yet made the difficult political calls — new rules to limit what can be built where on the shoreline.

“Even though sea level rise is something people think is going to happen in the distant future, it’s happening today,” said Steve Goldbeck, chief deputy director of the commission. “When you couple it with extreme storms and king tides, we could have serious flooding. We need to plan now.”

Contact Paul Rogers at 408- 920- 5045.

STAFF FILE PHOTO

Areas such as San Francisco International Airport that were built on fill that was not densely compacted are sinking nearly half an inch a year, a new study says.